Organic compound contaminants, especially volatile and volatilizable organic compounds (VOCs) in the environment may be hazardous to public health even at very low concentrations, since many of the VOCs are toxic, mutagenic, and/or carcinogenic, such as aromatic and halogenated compounds. Organic compound contaminants which are "volatile", as that term is used in the present invention, are those which have a relatively high vapor pressure and can be found in vapor form at relatively low temperatures. However, there is also included within the definition of "volatile organic compounds" (VOCs), as that term is used in the present invention, organic compounds which are "volatilizable", i.e., capable of being made volatile. Such volatilizable organic compounds are particularly those which may vaporize under the conditions of concentration and detection to be found during the methods of the present invention. The measurement of VOCs in air has become a very important goal. Conventional VOCs monitoring methods involve collecting a sample at the site and transporting it to the laboratory for analysis. While these methods are quite accurate, they cannot be utilized for continuous on-line analysis to provide information on a real-time basis as required for effective pollution control and for meeting regulatory requirements. A discussion of such methods, and of the state of the art relating to them, can be found in U.S. Pat. No. 5,435,169, which is incorporated herein by reference in its entirety.
In accordance with the present invention, analytical apparatus and instrumentation is provided which can be used in various aspects of continuous on-line measurement of organic compound contaminants, and which can also be used off-line for isolated, i.e., discrete measurements which may be single instances or repetitive occurrences. The objective of the analytical apparatus and instrumentation is three-fold. The organic compound contaminants are to be concentrated to facilitate detection of low concentrations; interfering species such as CO.sub.2 and H.sub.2 O are to be eliminated; and then the trapped organic compound contaminants are to be injected into the detector. A variety of detector systems may be used with this analytical approach, e.g., non-methane organic carbon analysis (NMOC), total organic carbon analysis (TOC), mass spectrometry (MS), infrared spectroscopy (FIR), or any other suitable detection system. Systems of this type can be used on-line to monitor emissions from industrial stacks, vents and similar sites from which emissions originate.
Total Organic and Non-Methane Organic Carbon Analysis--Total organic carbon is a measure of total carbon emissions in organic form, i.e., the total carbon content less that derived from the permanent gases such as CO.sub.2 and CO. Non-methane organic carbon (NMOC) is another category of organic compound contaminant measurement frequently used in addition to total organic carbon, and is a measure of the total organic carbon content of a sample, except that coming from methane. In non-methane organic carbon analysis, methane, CH.sub.4, is treated as a permanent gas, although it is not treated as a permanent gas in the other analyses.
In the mid-1970's, EPA Standard Method 25 was developed for quantifying NMOC emissions from stationary sources. In that method, gas samples are collected and sent to a lab for analysis. In a conventional non-methane organic carbon analyzer, one milliliter of gas sample is introduced into a separation column a through a gas sampling valve. The column is used to separate VOCs from permanent gases such as CO.sub.2, CH.sub.4 and CO. After the gases elute from the column, i.e., a CO.sub.2 peak appears, the column is backflushed into the detector system and all of the organics are then measured together as one peak.
The detection system comprises an oxidation unit, a reduction unit and a flame ionization detector (FID). The reason for converting all of the organic compounds to CH.sub.4 is that different compounds have different response factors in FID, and in this manner a response directly proportional to the number of carbon atoms is obtained.
The use of column separation in conventional NMOC analysis poses significant problems especially when the sample contains large amounts of moisture and the concentration of CO.sub.2 is above 8% by volume of the sample. Another major problem is that the detection limits are not low enough, as a result of the fact that the injection volume must be limited in order to obtain good separation in the column. Another drawback of this method is that it cannot be used for continuous on-line monitoring. Other total organic carbon analysis methods are also used where, instead of reducing the CO.sub.2 to methane, the CO.sub.2 itself is measured using infrared or other suitable detection means.
Continuous On-Line FID, MS and FTIR--At present on-line analysis is done using a flame ionization detector (FID) for total hydrocarbon analysis. Similarly, the mass spectrometer (MS) and the Fourier Transform. Infrared Spectrophotometer (FTIR) are used for on-line VOCs monitoring. In the case of both the FID and the MS, the sample is introduced directly into the detector. No sample concentration is used, and thus the detection limits are quite high. However, H.sub.2 O, CO and CQ.sub.2, which are always present in environmental emissions, interfere in the analysis. In the case of the FTIR, the absorbance spectra is measured in a flow cell, or else a long path FTIR is used in which the IR beam is reflected across the emission source. Here also, the presence of H.sub.2 O, CO and CO.sub.2 can also interfere with the analysis.
Monitoring VOCs in Water--Most conventional VOCs monitoring is done by using the purge and trap method. Typically, the sample is collected in the field and then transported to the laboratory. On-line purge and trap systems have also been developed for semi-continuous monitoring.